1. Field of the Invention
The present invention relates to a honeycomb structure used for a filter for trapping particles contained in exhaust gas from an internal combustion engine, boiler, or the like, and a method of manufacturing the same.
2. Description of Related Art
Since a large amount of particulates (particulate matter) containing carbon as the major component, which may cause environmental pollution, is contained in exhaust gas discharged from an internal combustion engine such as a diesel engine, a filter (diesel particulate filter (DPF)) for trapping particulates may be provided to the exhaust system of an internal combustion engine.
As a filter used for such a purpose, as shown in FIGS. 10(a) and 10(b), a honeycomb structure 1 having a plurality of cells (through channels) 9 partitioned by porous partition walls 7 and formed through the honeycomb structure 1 in the axial direction is generally used, in which one end of predetermined cells 9a is plugged with a plugging portion 11 and the remaining cells 9b are plugged at the other end opposite to that of the predetermined cells 9a, alternately.
Exhaust gas flows into a filter including the honeycomb structure 1 from one end face 3 and is discharged from the other end face 5 after removal of particulates contained in the gas. In more detail, exhaust gas flows into the cells 9b, of which the end is not plugged at one end face 3 of the filter and is plugged at the other end face 5, passes through the porous partition walls 7, moves to the cells 9a, of which the end is plugged at the end face 3 and is not plugged at the other end face 5, and is discharged from the cells 9a. In this case, the partition walls 7 function as filtration layers, and particulates contained in the gas are trapped by and deposited on the partition walls 7.
After using the filter for a predetermined period of time, particulates deposited on the partition walls of the filter are heated and removed through combustion by supplying electricity to an electric heater provided thereto or the like.
However, the temperature of the peripheral portion of the filter in the diametrical direction tends to be decreased due to a small amount of exhaust gas passing through those areas and a large degree of heat radiation to the atmosphere. Therefore, since particulates remain unburned in this area after regeneration of the filter due to insufficient oxidation and combustion of particulates, the partition walls of the filter are clogged. Moreover, in the case where unburned particulates are suddenly combusted during particulate combustion, the temperature of this area is significantly increased, whereby the filter tends to be eroded or damaged (due to thermal stress).
In order to prevent occurrence of uneven deposition of particulates inside the filter, a honeycomb structure of which the peripheral portion in the diametrical direction of the filter is plugged has been mainly used, for example. As shown in FIGS. 8(a) to (d), a method of manufacturing such a honeycomb structure includes (1) providing plugging portions to a honeycomb formed product, in which a plurality of cells communicating between two open ends are partitioned and formed by porous partition walls, in a checkered flag pattern at one end and the other end of the honeycomb formed product alternately so that either of the two open ends of each cell is plugged with a plugging material alternately, and firing the honeycomb formed product to obtain a honeycomb structure 40, (2) plugging the peripheral portion of the resulting honeycomb structure 40 in the diametrical direction with a plugging material in a predetermined range, and firing the honeycomb structure 40 to obtain a peripherally-plugged honeycomb structure 42, (3) grinding the periphery portions of the resulting honeycomb structure 42 in a predetermined dimension, and (4) coating thus ground honeycomb structure 42 with a coating material (material containing ceramics) 46 in order to form a protective layer on the periphery of the honeycomb structure 42, and drying the coating material to obtain a honeycomb structure 50.
However, since it is difficult to align the above honeycomb structure during peripheral grinding, the peripheral portion of the honeycomb structure in the diametrical direction cannot be formed to conform to the design. In recent years, an improvement of fuel consumption has been demanded. This demand may be satisfied by reducing the pressure loss by increasing the porosity of the filter. However, if the porosity is increased, the cells as fluid channels may be deformed when forming the honeycomb structure, or the degree of roundness of the outer diameter is decreased, whereby it becomes more difficult to align the honeycomb structure during peripheral grinding. Therefore, the yield is decreased when manufacturing a honeycomb structure peripherally plugged with a plugging material as in a conventional honeycomb structure. Moreover, since it is necessary to perform the firing step at least twice, the manufacturing cost is increased.